The numerical model has been validated with experimental data. An experimental vertical cylindrical latent thermal energy storage unit has been constructed and series of temperature measurements have been performed. Experimental test unit and thermocouples position inside the unit are shown in Fig. 2 and Fig. 3.

Fig. 3. Thermocuples position

An experimental test unit has been made of two concentric tubes, where the inside tube (0.033 m i. d., 0.035 m o. d. and 1 m length) has been made of copper, while the outside tube (0.128 m i. d., 0.133 m o. d. and 1 m length) has been made of brass. The outside tube has been well thermally insulated to reduce the heat losses. Sixteen K-type thermocouples have been placed inside the PCM at various locations. Two additional thermocouples have been placed at the inlet and outlet of the HTF into inside tube. All thermocouples have been connected to a data acquisition system. Labview commercial software has been used to record data in a database format on the personal computer. Temperature measurements have been recorded at a time intervals of 10 s. To maintain the axisymmetric melting around the inside tube, the test unit has been oriented in a vertical direction. Commercial paraffin Rubitherm RT 30, with thermophysical properties in Table 1, has been used in experimental studies as PCM, and water has been used as HTF.

Series of melting and solidification experiments has been performed for different mass flow rates and inlet temperatures of HTF. Computational model has been set up to reproduce these experimental conditions. Numerical calculations have been carried out for a grid size of 250 (axial) and 73 (radial) nodes, and dimensionless time steps of 0.06. In Figs. 4 and 5 paraffin temperature histories at locations 5 (x = 0.35 m; r = 0.0265 m) and 9 (x = 0.65 m; r = 0.0265 m) during melting, as well as at locations 2 (x = 0.05 m; r = 0.0355 m) and 15 (x = 0.95 m; r = 0.0445 m) during solidification are shown for both experiment and simulation. Mass flow rates and inlet temperatures of the HTF as well as initial temperatures of the PCM are indicated in figures.

The comparison between numerical predictions of time-wise temperature variations and experimental data shows a good agreement, although the natural convection in the liquid phase of the PCM has been ignored in the numerical model. It can be seen from Fig. 4 that melting of the applied PCM occurred non-isothermally over a certain temperature range within the melting zone. The shape of the temperature curves indicates that the melting dominates at about 27.7 to 35 oC. During solidification, paraffin has an isothermal phase change temperature range and no subcooling property, as shown in Fig. 5.

Fig. 4. Experimental and numerical time-wise PCM temperature variations during melting

The results of analysis have signified that a developed numerical procedure could be efficiently used to simulate thermal behaviour of the latent thermal energy storage unit during charging and discharging.